Nitrates link to cancer questioned
A medical adviser for Bowel Cancer NZ has criticised studies linking nitrate levels in drinking water with an increased risk of bowel cancer.
A major Danish study published in 2018 found a significant increase in bowel cancer when nitrate levels were just 0.87 milligrams per litre of water and a
15 per cent increase at 2.1mg per litre.
The current safe level in New Zealand, as mandated by the World Health Organisation (WHO), is
11mg per litre.
A new analysis by Otago and Victoria universities of overseas studies, including the Danish study, suggested between 300,000 and 800,000 Kiwis might be exposed to potentially harmful levels of nitrates in drinking water that could lead to an increased chance of developing bowel cancer.
Bowel Cancer NZ medical adviser Professor Frank Frizelle said of seven studies undertaken around the world on the same issue, four showed no significant difference, two indicated a slight increase in risk and the Danish study showed a notable increase in bowel cancer.
Frizelle said when analysis of all seven studies was measured against the number of bowel cancer cases recorded, it was questionable that there was ‘‘anything there at all’’.
He cautioned against overinterpreting the research, ‘‘particularly in the environment we are in where we have a big antidairy lobby and water purity lobby who want to throw everything they can on the fire to say it is causing all this damage’’.
It was clear nitrates were more common in rural water and bowel cancer rates were slightly higher in the same areas, he said.
‘‘That could be because rural communities eat more meat, which we know has a clear association (to bowel cancer).
‘‘What we have got now is a very loose association with nitrates and bowel cancer, and perhaps it doesn’t exist at all ... there is no logical reason or cause and effect.’’
The release of results from the New Zealand study has left some Selwyn residents uneasy.
The district has been under the microscope because of high nitrogen levels that Environment Canterbury
has blamed on farming intensification.
Selwyn District Council water quality test results show a sharp increase in nitrate levels in some areas over the past nine years, particularly in and around Rolleston.
Much of the district has levels above the 0.87mg per litre the Danish study suggested was the risk marker.
Council group manager of infrastructure Murray Washington said residents could be assured current testing showed nitrate levels in all Selwyn water supplies were well below the maximum levels for safe drinking water set by the Ministry of Health.
At a minimum, nitrate levels were tested annually, Washington said, increased to monthly if any results are above half the maximum level.
Water quality advocate and Springston resident Mike Glover stopped drinking council-supplied water in 2019 after the Danish study was released.
Testing on his property showed nitrate levels had increased tenfold in just seven years, from 0.48mg per litre to 4.48mg.
Last year he held a testing session at the Springston hall for concerned residents and was surprised to see 100 people turn up.
Using a professional nitrate tester, he tested 80 samples and delivered some shocking results, including a 11.5mg per litre reading near Dunsandel.
Ongoing pressure on the Ministry of Health from public health officials, environmental campaigners and local politicians has led to the establishment of a taskforce led by scientists who will examine possible links between nitrates in groundwater and cancer.